The invention relates generally to power converters and more specifically to multilevel configurations of converters.
The utility and importance of power conversion has grown with advantages being achieved in applications such as motor drives, renewable energy systems, and high voltage direct current (HVDC) systems, for example. The multilevel converter is emerging as a promising power conversion technology for various medium and high voltage applications.
Multilevel converters offer several advantages over conventional two-level converters. For example, the power quality and efficiency of the multilevel converter is better than that of the two level converter. Also, multilevel converters are ideal for interfacing between a grid and renewable energy sources such as photovoltaic (PV) cells, fuel cells, and wind turbines.
Transformer-less multilevel converters have been designed using a modular structure. Such multilevel converters typically include a plurality of power modules that are coupled between DC buses. The modular structure of the converters allows stacking of these converters to provide different power and voltage levels.
In some modular multilevel converter embodiments, the power modules are controlled to control voltage observed at output terminals of the converters. Typically each power module contributes to one step of voltage control in the converter. In symmetric configurations, the number of power modules required to develop a converter of a desired voltage rating can be calculated by taking into consideration the voltage rating of the converter and the voltage step change required for control of the power converter.
For example, in symmetric configurations for a power converter with a voltage rating of 10 kV, and a voltage step of 1 kV, at least 10 1 kV power modules would be required to achieve the desired result. The number of power modules required to design desired power converters in a symmetric configuration thus increases with an increase in power converter voltage ratings.
The increase in number of power modules leads to increase in cost of the power converter. Further, an increase in power modules leads to reduction in efficiency of the power converter.
It is also possible to increase the voltage step change. In this case, fewer modules may be required to develop the desired voltage rating, but for the same average switching frequency, there will be an increase in the resulting ac voltage harmonics.
Hence there is a need for modular multi-level power converter systems that provide for the same voltage rating and harmonic performance as symmetric converters while reducing the number of power modules used.